Summary The orchestration of growth and differentiation during development is essential for normal mechanical and physiological function of organs and appendages. Despite its importance, the regulation of how coordinated growth occurs such that proportion is maintained is not well understood. To gain insight into this process, we studied the congenital overgrowth disorder macrodactyly characterized by local giganticism of the digits of the hand and feet. Macrodactyly is a unique growth disorder in that the skeletal and soft tissue elements of the digits including nerve, vasculature, muscle and bone are enlarged, however organized such that larger digits form. Thus, patients with macrodactyly represent a unique biologic opportunity to explore fundamental mechanisms of how growth, size and proportion are coordinated within and between tissues. We have identified a specific somatic activating mutation in the catalytic subunit of phosphoinositide 3 kinase (PI3K), encoded by the gene PIK3CA, within affected tissue of macrodactyly patients. This mutation is also commonly found in cancer and other overgrowth disorders; thus, this mutation alone cannot explain the coordinated growth response observed in macrodactyly. How does this particular mutation lead to overgrowth that is coordinated and patterned during the development of the limbs? The restriction of overgrowth to the digits in these patients as opposed to the proximal limb, suggests a role for tissue interactions in the distal limb during development to regulate patterned growth and size. Additionally, we have identified potential modifier mutations arising in concert with PIK3CA that may be essential for PI3K regulation of overgrowth. We hypothesize that PI3K-mediated signaling coordinates growth through specific paracrine signaling from sensory neural tissues during development in a manner that is dependent on signaling between the apical ectodermal ridge (AER) and mesenchymal tissues during limb development. We propose to investigate the regulation of coordinated growth by PI3K through: 1) identification and localization of the affected cells and tissues containing mutant PIK3CA in macrodactyly patients and examination of the effect of these mutant cells on adjacent tissues; 2) use of novel zebrafish models to dissect the genetic and developmental causes of overgrowth though systematic analysis of newly identified secondary modifier mutations in regulating PIK3CA function and 3) the use of a conditional knockin mouse model of activated PIK3CA to test the sufficiency of increased PI3K signaling within specific tissues of the developing mouse limb to phenocopy macrodactyly. The proposed research will define the molecular and tissue level regulation of organ size and growth that will lead to new treatment modalities for a host of overgrowth and neoplastic conditions and yield new strategies for enhancing tissue repair and organ regeneration.